The overall goal is to specify environmental and endogenous factors that control seasonal rhythms of mammals. We seek to 1) characterize a novel melatonin-independent seasonal timing mechanism and its relation to the melatonin-dependent interval timer 2) specify which melatonin target tissues in the diencephalon control seasonal rhythms of behavior 3) determine whether the dorsomedial hypothalamic nucleus mediates seasonal changes in sensitivity to steroid hormones and 4) identify the hypothalamic substrate essential for encoding day length information. Syrian and Turkish hamsters are the model species used in this research. By means of peripheral and central hormone administration and selective ablations of melatonin target tissues we hope to determine the central nervous system substrates for seasonal rhythms. Seasonal mood changes, suicide, changes in appetite and activity levels are but a few of the well known examples of seasonal rhythms in human behavior. The proximate causes of human rhythms remain largely unknown. The proposed research will help establish a conceptual framework for investigations of human chronobiology. Studies of photoperiodic control of seasonal rhythms also address a fundamental question in regulatory biology; seasonal timing is crucial to individual survival and reproduction of most vertebrates and an understanding of these phenomena is of potential value in controlling and preserving animal populations.